1. Field of the Invention
The present invention relates to a non-contact position sensor for sensing changes in magnetic flux and converting the changes to a voltage, and in particular relates to a sensor employing a stator with three magnet facing sides that face magnets.
2. Description of the Related Art
Rotary sensors and linear sensors are examples of non-contact position sensors employing a hall element. FIG. 1 is a cross-sectional view of the center of a rotary sensor disclosed in Japanese Laid-open Publication No. Hei. 8-35809. As shown in FIG. 1, a coupling shaft 1 has a cylindrical rotor 2 on an outer side, with a cylindrical permanent magnet 3 being fixed to the inner side of the rotor 2. Two semi-cylindrical stators 4 and 5 are then fixed at the inner side of the permanent magnet 3 in such a manner as to leave a slight gap with respect to the permanent magnet 3. A gap 6 extending diameter-wise is formed between the stators 4 and 5 and a hall element 7 is provided in the gap 6.
When the cylindrical rotor 2 and the permanent magnet 3 rotate about the periphery of the stators 4 and 5, the magnetic flux passing through the hall element 7 changes. This change in magnetic flux is reflected as a change in the voltage of the hall element 7. The angle of rotation of the rotor 2 can then be measured by measuring the change in this voltage.
FIGS. 2(A), 2(B) and 2(C) are views showing a configuration for a linear sensor disclosed in Japanese Patent Publication No. Hei. 7-500421. This sensor comprises a hall element 11, stators 12 and 13 provided so as to sandwich the hall element 11, and a moveable magnet 14, with a further stator 15 being provided on the opposite side to the stators 12 and 13 of the moveable magnet 14.
The moveable magnet 14 is fitted to a slider 16, with the slider 16 moving in a lengthwise direction. When the moveable magnet 14 is located between the stators 12 and 13 as shown in FIG. 2(A), the two magnetic circuits A and B are formed in a symmetrical manner, so that magnetic flux passing through a hall element 11 located centrally becomes zero. When the moveable magnet 14 moves to the right as shown by FIG. 2(B), the balance between the magnetic circuits A and B breaks down and the hall element 11 becomes included in the magnetic circuit A. Changes in magnetic flux therebetween are then sensed and lengthwise movement of the slider 16 can be known. When the moveable magnet 14 moves to the left as shown by FIG. 2(C), the hall element 11 is included in magnetic circuit B and the position of the slider 16 can be detected from changes in magnetic flux therebetween.
The rotary sensor of FIG. 1 employs a toroidal permanent magnet. The output gradient is then set so that the output characteristic angle has a 180° cycle. However, only approximately 150° of the output characteristic angle is usable, with resolution falling accordingly when less than 150° is used.
On the other hand, with the linear sensor of FIGS. 2(A), 2(B) and 2(C), when the thickness of the moveable magnet 14 is made large, the distance between the stators 12 and 13, and the stator 15 becomes large, leakage of magnetic flux increases, and the linearity of the output value becomes poor. When the thickness t is made small in order to avoid this, magnetic force becomes weak and a desired sensitivity cannot be obtained.